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FLEXO Magazine : March 2014
PLANTS & PROCESSES Doctor Blades: Commodity Or Key Process Element? When Does it Make Sense to Buy on Price? By Paul Sharkey Doctor blades began replacing rubber metering rolls in the flexographic ink transfer process in the early 1970s. Compared to metering rolls, doctor blades could sheer ink from an anilox surface more precisely. They were also less affected by the hydraulic pressure of ink that widened the gap between the anilox and metering roll. In the beginning, ink was first delivered from an ink pan to the anilox by a rubber fountain roller. The doctor blade re- placed the metering roller and wiped excess ink from the an- ilox in a positive “ wiping” position. In this position, the blade was subject to the same hydraulic pressure as the metering roller. Press speeds were restricted by an increase in hydrau- lic lift. Soon the doctor blade was reversed, so as to engage the anilox in a negative or sheering position; which resulted in reducing the anilox ink film even at higher speeds. At that time, the only downside to doctor blade ink metering was an increase in damage and wear to the mechanically engraved and chrome plated anilox surface. Some anilox wore flat in a matter of weeks. In the1970s, doctor blade specifications for a typical flexo printer were: • Material Hardened and tempered blue strip steel • Thickness Thin as possible, so as to reduce the contact area with the anilox surface—usually 0.006 -in. thick • Edge None specified—it was often a ripped or rough edge Doctor blade material was often purchased from the cheapest local source of commercially available strip steel— the same material was used for strapping and banding wood and industrial products. Chrome anilox suppliers applauded the move to doctor blade metering and the resulting boon in their reconditioning business. In the 1980s, ink metering changed radically for two reasons: • Laser engraved anilox rolls were developed. They were far more wear resistant than a mechanically engraved chrome surface and made it possible to engrave a much smaller and higher line count. Plus, the surface ink film could be reduced, so it became possible to print finer type and smaller dots with less dot gain • Chambered doctor blade metering systems were introduced. They lowered the amount of ink required to charge the system by as much as 50 percent. Ink was less vulnerable to evaporation and atmospheric contami- nation and more stable for longer runs Laser engraved anilox rolls, and to some degree cham- bered ink metering systems, opened the door to the many key process element improvements that followed. Think about it: Improvements to inks, plates, mounting tapes, sleeves, doctor blades and so on were not needed when an anilox line count was restricted to 360 cells per inch or less. As a result, flexo evolved to be a specified print method for many quality conscious print buyers. DOCTOR BLADE EVOLUTION When laser anilox rolls were first introduced, the cost of each was often five times greater than the chrome surface it was replacing. Printers applauded the quality gains resulting from higher line counts and thinner ink films, but the laser surface was vulnerable to wear and damage. This vulnerabil- ity initially slowed migration to laser surfaces. In response to concerns about wear and damage, new types of doctor blade steels and new blade edges were developed. DOCTOR BLADES: A RETROSPECTIVE • 1970s The doctor blade replaced the metering roller and wiped excess ink from the anilox in a positive “ wiping” position • 1980s Doctor blades evolved in response to development of laser engraved anilox rolls alongside the introduction of chambered metering systems • 1990s Standard strip steel blade materials arrive on scene • Modern Day HD strip steel launches dawn of a modern era SECOND INA SERIES From left to right: Available since the 1970s with low cost, low carbide structure and little wear resistance; available since the 1990s with standard strip steel and moderate wear resistance; available since 2012 with new HD strip steel, high wear resistance and low anilox surface impact 30 FLEXO MARCH 2014 www.flexography.org